Sticky But Slick: Reducing Friction Using Associative And Nonassociative Polymer Lubricant Additives

Sustaining the current trend toward lower viscosity lubricants requires the development of improved friction modifier additives to compensate for the increased prevalence of boundary lubrication. In this work, we demonstrate that functionalizing oil-soluble olefin copolymers (OCP) with a random distribution of a small amount (around 1.4 mol %) of polar-aromatic groups provides an additive with potential for enhanced boundary lubrication. Extensional relaxation time measurements show that the functionalized olefin copolymer (FOCP) can form transient, associative bonds between polymer chains, while quartz crystal microbalance with dissipation monitoring (QCM-D) demonstrates that the amphiphilic nature of FOCP drives adsorption at the solid-liquid interface. Colloid probe atomic force microscope (AFM) measurements show that the adsorbed layer forms a steric barrier that reduces friction in the boundary regime. Comparing interactions between preadsorbed FOCP layers in bulk solution with and without FOCP chains reveals that FOCP chains in the solution interact with the adsorbed polymer to increase interfacial viscosity, important for promoting hydrodynamic lubrication, while also promoting adhesive bridges. Adhesive bridging between adsorbed FOCP layers is reduced when the FOCP-containing solution is substituted by one containing only OCP, which further reduces friction. Results reveal a compromise between adsorption and adhesion in boundary lubrication by FOCP. This mechanistic insight provides a platform for the investigation and development of functionalized polymeric friction modifiers.